A Fundamental Principle of Aeronautical Engineering Has Been Overturned (wired.com) 112
An anonymous reader quotes a report from Wired: Aerodynamic drag is a major "barrier" in high-speed airplanes, automobiles, and bullet trains. This is because a design with less aerodynamic drag allows the aircraft to move at higher speeds with less energy. When an aircraft or car body moves at high speed, a thin layer of air called the "boundary layer" is formed on its surface. This boundary layer has two states: laminar flow, in which air flows in an orderly fashion, and turbulent flow, which involves turbulence. The longer the air stays in the laminar flow state with low friction, the smaller the air resistance becomes, but as the air speed increases, it transitions to turbulent flow. The key to reducing aerodynamic drag is how to delay this transition to turbulence.
For more than 80 years, the principle of "the surface of an object must be smooth" has been the basic premise of aeronautical engineering throughout the world in order to suppress the transition to turbulence and reduce aerodynamic drag. This premise was based on the results of a 1940 study by Ichiro Tani, a Japanese aerodynamicist who quantitatively demonstrated the relationship between "surface roughness" (an indicator of the state of the machined surface) and turbulent transition, arguing that surface roughness, which was unavoidable with the manufacturing technology of the time, prevented laminar flow from being realized. However, in 1989 Tani reinterpreted the experimental data on rough-surface pipes obtained by fluid engineer Johann Nikulase in the 1930s, bringing a new perspective that "roughness may not necessarily only promote turbulent transition and increase fluid resistance." Inheriting this idea, a research group led by Yasuaki Kohama of Tohoku University experimentally demonstrated in the 1990s that fibrous rough surfaces, which have fine fibrous irregularities on their surface, have the effect of delaying transition under certain conditions.
The same Tohoku University research team recently announced a discovery that significantly advances this trend. Aiko Yakino, associate professor at Tohoku University's Institute of Fluid Science, and her research group were the first in the world to demonstrate that aerodynamic drag can be reduced by up to 43.6 percent simply by applying distributed micro-roughness (DMR), a surface roughness so fine and irregular that it cannot be distinguished by the naked eye. This technology is fundamentally different from the "rivulet (shark skin) process," which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.
For more than 80 years, the principle of "the surface of an object must be smooth" has been the basic premise of aeronautical engineering throughout the world in order to suppress the transition to turbulence and reduce aerodynamic drag. This premise was based on the results of a 1940 study by Ichiro Tani, a Japanese aerodynamicist who quantitatively demonstrated the relationship between "surface roughness" (an indicator of the state of the machined surface) and turbulent transition, arguing that surface roughness, which was unavoidable with the manufacturing technology of the time, prevented laminar flow from being realized. However, in 1989 Tani reinterpreted the experimental data on rough-surface pipes obtained by fluid engineer Johann Nikulase in the 1930s, bringing a new perspective that "roughness may not necessarily only promote turbulent transition and increase fluid resistance." Inheriting this idea, a research group led by Yasuaki Kohama of Tohoku University experimentally demonstrated in the 1990s that fibrous rough surfaces, which have fine fibrous irregularities on their surface, have the effect of delaying transition under certain conditions.
The same Tohoku University research team recently announced a discovery that significantly advances this trend. Aiko Yakino, associate professor at Tohoku University's Institute of Fluid Science, and her research group were the first in the world to demonstrate that aerodynamic drag can be reduced by up to 43.6 percent simply by applying distributed micro-roughness (DMR), a surface roughness so fine and irregular that it cannot be distinguished by the naked eye. This technology is fundamentally different from the "rivulet (shark skin) process," which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.
Mythbusters? (Score:4, Informative)
Re:Mythbusters? (Score:5, Interesting)
The article even mentions that there's other surface irregularities that decrease friction, and the paper mentions other methods that also decrease drag, but the newly observed effect is distinct and different with the ones we've seen so far, and the headline is reductionist and bad.
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Good point - reminds me I have a couple of case fans that are meant to be super quiet and have dimples on the blades, presumably to that end.
I'm no aerodynamics engineer but a relationship between reduced drag, and quietness, seems reasonable.
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Re:Mythbusters? (Score:5, Informative)
The dimples on golf balls are actually to create turbulent flow. TL/DR, a sphere isn't a very aerodynamic shape; its rear taper is too sharp, so flow detaches and there's a big low pressure wake in the back. High pressure in the front and low pressure in the rear = pressure differential, and a large area times the pressure differential = large drag force.
While it's best to not have flow separation, or at least delay it as long as possible, if you're going to have flow separation, you commonly want to generate vortices at the point of flow separation. That's why cars commonly abruptly truncate (kammback) where they'd become too steep in the rear rather than continuing to curve, and often have various vortex generating surfaces (lips, radial protrusions, etc) at the termination; it causes air to "pull down" and help fill in the wake. This is what the dimples on golf balls do.
Now, most of the dimples on a golf ball at any time are actually doing harm, or at least not helping. You really only want the dimples right around the point of flow separation. Unfortunately, golf balls don't have a specific flight orientation, so it's all or nothing - and "all" happens to be the better choice.
But as mentioned, this is entirely different than what is being talked about here, which is about the laminar-turbulent flow transition.
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A larger boundary layer also increases the Magnus effect which generates lift from the ball's back-spin.
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So basically what you are saying is that the Mythbusters could have gotten at least as good a result, if not better result, by only having dimples on the car at the point of air flow separation? Like a ring of dimples around the whole car just in front of the front doors? And a second ring around the max curve of the grill area? [Just a 'for instance' - I have no idea if those are the correct areas. IINALAFE].
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I wonder how practical this is. It appears that the surface features need to be so small that they couldn't be painted over, for example. Paint adds significant weight to an airliner, but is worth it for the protection and performance it provides.
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This actually effectively "is" the paint. The only questions are about how durable it will be in flight conditions (two types are discussed, protrusions and dimples). Basically, paint with bumps vs. paint with nicks in it.
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There's also been recent research that shows that plasma actuators or electrohydrodynamic (EHD) flow control effectively eliminate drag. I'm not sure why this specific TFA gained publication but the ionic charge did not.
https://link.springer.com/article/10.1007/s40430-026-06357-y
Woah, cool (Score:5, Interesting)
It's not that surprising that someone found another exception to the the rule of thumb that's been proven wrong with many mechanisms at many scales, including the shark skin, but also just, golf ball dimples, and all kinda wacky methods on aircraft wing like shock bodies and all...
Re:Woah, cool (Score:4, Interesting)
TBH i was pretty surprised by that aspect. The article claims at parts up to 43% reduction in drag... that's pretty massive. I'm having a hard time seeing how flow around a meter scale object would be affected if it was suspended by a few sub-millimeter diameter wires.
The whole article is kinda hard to read, imo, and i suspect it's got this wrong, but don't know for sure. I appreciate that this is a difficult subject to explain simply, but they don't do a great job.
Tennis ball hair (Score:1)
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This has nothing at all to do with how dimples help golf balls [slashdot.org] (just the opposite, actually)
Hence it was not a "fundamental" principle (Score:5, Informative)
It was a generally accepted principle. Please stop with the abuse of language to make things sound more flashy. It just makes you look dumb.
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Hi, someone who regularly does CFD simulations in OpenFOAM here. It is a fundamental principle. I hope that helps.
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You are confused. And you should look up "argument from authority" and why a simulation is not reality and a fundamental principle within a simulation does not make that principle fundamental for the thing simulated.
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I have no idea. Why does it matter?
Re: Hence it was not a "fundamental" principle (Score:2)
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Yes. Quantum gravity is missing. And we have no idea what that means. It may be tiny adjustments at "normal" conditions, but there may also be massive surprises in there. There is also another thing: Whole Quantum theory has been experimentally verified, the precision of these verifications is massively lower than for any sane prediction that quantum computing will work. This is a second area for likely surprises.
The thing people often get wrong is that both relativity and quantum theory are _models_ of rea
Re: Hence it was not a "fundamental" principle (Score:2)
Not every model humans have come up with. Pamela Anderson in 90s Baywatch? No flaws there. And incomplete? Please, she was complete...
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What did textbooks call it?
Most likely they would refer to that as "standard engineering practice suggests" or similar.
You should try reading some textbooks yourself, they can be very educational.
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You should try reading some textbooks yourself, they can be very educational.
Boundless arrogance is boundless arrogance. I am a PhD engineer.
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And also an idiot.
Did you even click the right post to reply to?
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Suppose I replied to the wrong posting. How exactly do you think the quote of your posting did get in there? Right. You obviously did not think.
I would say "takes one to know one", but I would not know what being an idiot feels like.
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Thanks for pointing that out. It is, obviously, a rhetorical question though. These idiots replace facts and insight with "how they feel" and then apparently expect reality to conform to that.
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Thanks for the reference.
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What did textbooks call it?
It's been a while, but IIRC none of my textbooks called things fundamental concepts; rather, it simply referred to the impact of various things on things like drag, lift, stability, etc. The only fundamental principal I recall was the basics of flight - Stall, Spin, Crash and Burn...
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No surprise. Textbooks tend to not call things fundamental that are not. Especially as there are better words, like "standard approach", "best understood approach", etc.
Re:The Profit Effect. (Score:4, Insightful)
Yes. I can actually say the word suicide without some moronic need to use idiotic terms like “unalive”. Sexual assault is a valid term and legal description for a horrific crime. Not a written prescription to dismiss the “essaaay” away because it makes you unprofitable to say.
This kind of thing comes from American Puritanism on the internet.
Most countries can handle the word "death" or "dead" being used but the US for some reason cant so American companies move to automatically censor these words. Just look at BBC programming, Top Gear presenters regularly swore (oh cock) and this was screened at 8 PM on a Sunday (one of the most popular time slots on UK TV). I expect this to get better as the US loses relevance.
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That is the opposite though. The judge in the 1973 case was removed from office and it's not hard to see why; he was a moron:
Petitioner was found to have prodded a deputy public defender with a "dildo" during a conference in chambers one morning, and later that day to have referred to this incident twice in open court so as to curtail the victim's cross-examination of two witnesses. Petitioner was found to have approached a court commissioner from behind in a public corridor of the hall of justice and to have grabbed this victim's testicles. Petitioner was found on two occasions to have made lustful references to his female clerk, once while in chambers in the presence of a group of professional associates. Petitioner was found to have habitually used vulgar and profane language in his conversations with this clerk, and on two occasions to have used profane terms of personal abuse in reprimanding her and another woman employed by the court. Petitioner was also found to have invited two female attorneys into his chambers wherein he discoursed on the salacious nature of the evidence adduced in criminal cases concerning homosexual acts and rape, punctuating his commentary with profane terms for bodily functions.
I don't think it's an absurd application of puritanism to remove someone who is manifestly unfit for office. This guy genuinely disrupted the judicial process with his behaviour. It's not the same thing.
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I don't think you can pin our cultures issues with death on Puritanism. Strip away everything else and at its root Puritanism is a Christian sect. Christianity is a faith that is quite concerned with what death is, its implications, and gives a lot of consideration to implications of the death the Messiah and the gruesome circumstances of it.
I do agree as an American our culture has developed a very odd relationship with the reality of death, I don't think you blame Christianity for it. In fact I think the
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By all means attempt to unpack the strange relationship American culture has with death but do it without the Christophic bias..
Looking at effects of Christian religions is not "Christophic". Claiming so is fundamentally dishonest and an attempt to shut down a factual discussion.
Now here is a fact: All religion is made-up. And many of the followers understand that on some level. They just do not want to be reminded of it, as they (mis-) use religion for things like dealing with existential terror.
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This kind of thing comes from American Puritanism on the internet.
Well, the last piece of that sentence is the key part. I, and most everyone I know, have no problem using English language words. It seems to me that the only reason to censor yourself with weird, invented terms is because you want to be able to monetize your content on YouTube.
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This kind of thing comes from American Puritanism on the internet.
Most countries can handle the word "death" or "dead" being used but the US for some reason cant, so American companies move to automatically censor these words.
My guess is that this is the deeply religious that deep down know their certainties about death are imaginary. They really do not want to be reminded of that.
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As an American citizen who remembers decades of normal jargon being used in broadcast TV and shows, it’s painfully obvious the phenomenon is driven by social media profitability, not Puritanism. America could say these words just fine less than a decade ago. Back before we invented such stupidity like “unalive”. Self-censorship became as natural as not cussing your fucking face off in the boardroom. Only far less justified.
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I doubt many young people can define what an actual âoeracistâ is now that the term is some kind of acceptable retort in casual debate.
It's acceptable where it's applicable, and there's a shitload of racists out there. Defining "racist" is now a politically complex process. The word used to mean someone who believes that race is real and who further believes that some races are inherently genetically superior to others. The only useful distinction is that "a racist" (as opposed to someone who "is racist") should only be applied to people who are willfully perpetuating racism, not to suggest that only privileged people can be "racist" which
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Sorry, but in a somewhat fuzzy sense, "race" _is_ real. It just does not mean what people that try to elevate themselves by their racial membership think it means. For example, Science says no impact on intelligence. But try to ask a skin-doctor whether he can treat black and white people simply the same. (Obviously, this is more complex than just black vs. white.) There are countless medical effects from racial membership. Note that racial membership can be stronger or weaker or mixed. But it has medical e
Nonsense (Score:4, Interesting)
"43.6% reduction in surface air resistance with a fine roughness pattern of 1.0% (convex patterns of 38 to 53 micrometers)"
The picture of their wind tunnel is pretty great [tohoku.ac.jp]. They use magnetism to suspend the object, so the supports don't distort the airflow.
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Agreed. Deliberate adding of surface roughness (and even multiple surfaces) have been used in racing yachts for many decades. I think they started doing this in the 90s.
ha! (Score:4, Funny)
I told you guys that whole Bernoulli thing was total BS .. wait .. what, this wasn't that? Nevermind.
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Well, it is true that the wing attack angle generates 2-3x more lift, than a cambered wing at 0 degrees. So while both the attack angle and camber contribute to lift, the attack angle is far more significant. This is why some planes can fly upside down.
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This is why some planes can fly upside down
Intriguing: can you post some examples of aircraft which cannot generate enough lift when upside down?
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It's not about lift. Any airplane could theoretically fly upside down, in terms of lift, if the attack angle is right. Some planes cannot fly upside down because their engines aren't designed to operate upside down; fluids may depend on gravity for proper flow.
https://www.aerotime.aero/arti... [aerotime.aero]
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I told you guys that whole Bernoulli thing was total BS ..
But the lift from camber and from angle of attack are both caused by the Bernoulli effect.
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Correct. That's why I specifically noted camber, and didn't separate it from Bernoulli.
Depth? (Score:2)
How deep are the grooves?
Does a dirty car or airplane negate the benefit?
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I remember reading about the fight between polished aluminum planes and painted. The paint adds weight, and thus increases fuel consumption, but the paint lowers maintenance costs.
A dirty airplane can absolutely burn a noticeably larger amount of fuel.
A car is operating at much lower speeds, generally, so the effect is probably much less.
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A phrase to be heard either uttered on Slashdot in 2026, or at Woodstock ;)
But to be clear, the answer (assuming you're talking about sharkskin / riblets): a few dozen microns tall and a few dozen microns apart, with the individual riblets being very narrow, just a couple microns.
Sandpaper Smooth Fiberglass Sailboat Hulls (Score:5, Interesting)
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As per the principle in this article, that would make it worse if applied to the Reynolds regime in question. This is not about "general roughness", but specifically shaped roughness. In particular, a very sparse roughness on an otherwise smooth surface.
Sanding a hull is dealing with entirely different things. Sanding in general first off gets rid of microprotrusions and broader undulations. There is no question that this helps. The question to whether to polish to a matte or smooth surface is less obv
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Cruise ships blow bubbles out the sides of their hulls to reduce friction...I saw them and asked, learned it's a "thing". I believe the same thing is done on high speed torpedoes (though we're probably not supposed to know that)
(and I just *knew* shark skin would be entering this discussion)
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You're thinking of supercavitation. While it's something used for underwater vessels such as torpedoes, is it the same principle for partially submerged hulls?
https://en.wikipedia.org/wiki/... [wikipedia.org]
This has been applied in cycling.. (Score:3)
In the cycling world, much attention is paid to aerodynamics.
My Ridley Noah from around 2011 has strips of a rough tape (about 3mm across) in spots that sounds like what the OP is covering. There are also slits in the forks and seat stays that apparently have a beneficial effect on airflow, besides the more commonplace aerodynamic tube shapes. (Kammtail the standard now over more traditional aero shapes.)
They stopped using the tape after a few years, but have seen it pop up with other brands. I think the latest iteration has a very narrow indented 'line' instead. But things like dimpled wheels, and even 'whale fin' wheel designs make some interesting aerodynamic claims.
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Your blah-blah is refuted by the details of the summary, though.
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Nah it's the defeningly clicky freewheel mechanism that makes bikes fast.
Who would have thought? (Score:1)
Oh darn. (Score:3)
I guess I will have to give back my science fair ribbons from 1985 when I used the groove theory to try to prove that grooves from the leading to trailing edge of a wing improved its efficiency. I got all of the way to the state science fair with that project. Turns out there's a better way, and its golf-ball dimples.
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TFA (which is dense and not the clearest) addresses specifically that this effect is different from golf ball dimples and also from shark skin structures.
For how long have Golf balls had dimples? (Score:2)
And why? Tell the author, he might learn something new.
Two words (Score:1)
Golf ball
Re: Two words (Score:1)
Golf balls have you beat (Score:2)
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This comment section is jam packed full of people mentioning the same misconceived and inapplicable thing [slashdot.org], over and over. A thing that was actually discussed in the article, which they did not read.
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This comment section is jam packed full of people mentioning the same misconceived and inapplicable thing [slashdot.org], over and over. A thing that was actually discussed in the article, which they did not read.
You've clearly been around /. long enough to know that is a fundamental principle of /.; one that will never be disproven.
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This comment section is jam packed full of people mentioning the same misconceived and inapplicable thing [slashdot.org], over and over. A thing that was actually discussed in the article, which they did not read.
You've clearly been around /. long enough to know that is a fundamental principle of /.; one that will never be disproven.
I'm still holding out hope that AI and editors can create micro textured summaries that increase understanding, click-through, and overall comprehension by 43%
magnetic levitation optional
It's not scruffy, it's "aerodynamically enhanced"! (Score:3)
Wait, does this mean that all the tiny scratches on my car windscreen that I haven't bothered to polish out might actually be saving fuel?
Brb, getting a wire brush.
Roughness (Score:2)
An Incorrect Statement (Score:1)
Obligatory Simpsons (Score:2)
""These are speed holes. They make the car go faster."
Not holding my breath (Score:1)
Formula One (Score:1)
Horse Feathers (Score:2)
so feathers are better than skin or scales for flight surfaces, if only we could have learned this from elsewhere in nature
I have to wonder if they will find that a compliant fiberous surface is an improvement too
This doesn't surprise me at all (Score:2)
Before I got to the end of the first paragraph I thought of how a bit of surface roughness can reduce drag between two solids in contact with each other. The best example I can come up with involves applying pressure while dragging your finger across a glass surface. If the glass is smooth and shiny your skin tends to stick and sort of 'ratchets', while on a finely etched surface this doesn't happen.
I don't have the math or science chops to know if there's any theoretical connection between what I just desc
Turbulent flow.... (Score:2)
" which involves turbulence"
you don't say
Assertion dissonance (Score:3)
A 767 operates at Reynold's numbers between 30 and 40 million which is to right side of the curves in this paper where the with/without lines come back together.
Interesting, but the hyperbole is multidimensional.
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Uh, yeah you go swim up close to a shark to examine its skin, I'm sure he'd appreciate that.
Re:shark skin (Score:4, Interesting)
Re: shark skin (Score:2)
YMMV on this one, foo as he attempted to finish the post with his remaining hand.
Re: shark skin (Score:3)
I've dove in the presence of many full size tiger and bull sharks at once. You have to chum the water or they don't even come. People come into contact with them often and without incident, but you don't want to go out of your way to touch them. They can do a 180 in the blink of an eye.
Re: shark skin (Score:5, Insightful)
golf balls.
they are designed that way for a reason.
and are much more approachable than sharks
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Even the summary mentions that this is different to the "shark skin" effect. People have no attention span these days.
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Re:shark skin (Score:5, Interesting)
I think the only reading comprehension difficulty here is on your side. The impacts of "roughness" as a general term is a fundamental aspect of aerodynamic engineering. There has been evidence steadily emerging over time that this isn't exactly correct, that the distribution of roughness matters greatly, and the right distribution can even surpass a smooth surface. The confirmation in this paper helps close the chapter on this.
And honestly, their two approaches doesn't sound that difficult to manufacture at all. Certainly much easier than riblets. And the side effect of the first one - surface glass beads - would actually be beneficial for RAM. One of the principles for radar absorption is that you want a steady transition of the impedence (and by relation, dielectric constant) from the surface (which you want to be as much like air as possible) to the deeper layers. The outermost layer of RAM is commonly something like PTFE full of hollow glass beads. Under that you may have pure PTFE, and under a polymer with like 5% chopped carbon fibre fill, and so on. Well, here it turns out that having tiny glass beads on the surface can improve your drag coefficient as well.
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And honestly, their two approaches doesn't sound that difficult to manufacture at all.
Indeed.
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You should not have, because the researcher who claimed you should retracted his own claim in 1989. This story is the latest release of a rough example working better than smooth.
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the researcher who claimed you should retracted his own claim in 1989.
Interesting, I didn't know that.
Re:shark skin (Score:5, Informative)
The gold standard for laminar flow aircraft is gliders. Modern gliders can achieve laminar flow up to 95% of chord on the bottom surface, and around 40% on the top surface. Any waviness across the direction of the airflow of more than .004 inch can cause the boundary layer to detach, and form a separation bubble, at the rear of which the flow re attaches as turbulent flow. The bubble has a sluggish internal vortex.
Modern gliders place a spanwise turbulator on the bottom surface to force the turbulent transition before a bubble forms, either by mechanical strips of blowing air from small holes fed by a NACA duct.
The air sees the bubble as though it were a bump on the the surface, with the associated drag penalty.
Laminar flow is assisted by the acceleration of the airflow from the stagnation point on the leading edge to the point of maximum thickness of the airfoil. When the flow starts to decelerate, laminar flow is easily lost.
Turbulent flow thereafter has increased drag known as scrubbing drag on the surface, and the friction between the surrounding air and the turbulent flow.
A simple way to look at it is view a wing from the front at a zero angle of attack. The part of the surface you can see can support laminar flow.
Placing chordwise grooves may help to promote laminar flow a bit further, however surface contaminants will quickly render any gains moot. The flow is so easily disturbed that Splattered bugs on the leading edge can rob up to 30% of the drag reduction, so those same gliders have mechanical bug wipers that can clean the leading edge in flight.
Composite wings have a much better chance of laminar flow, in practice metal wing in service required frequent resurfacing to maintain laminar flow. Those with rivets, suck as the much vaunted P51 laminar wing rarely achieved laminar flow to any great extent doe to surface imperfections.
Further info and diagrams here: https://eaglepubs.erau.edu/int... [erau.edu]
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I have to disagree. While I agree the understanding of roughness impact has evolved over time; The summary clearly indicates Ichiro Tani introduced this idea in 1940 and retracted it based on further analysis of data [from 10 years prior to his claim] in 1989. The rest is just examples of people hunting down the best inverse examples of what he'd previously claimed... with this being the latest 2026 release directly descended from his own work.
It's actually impressive flex to claim an idea you pushed that m
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beat me to it, i was just gonna say some of these scientists kids finally made them watch sharkweek.
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Compare this to shark skin in water.
From TFS, FCS: "This technology is fundamentally different from the "rivulet (shark skin) process," which is known as a typical aerodynamic drag reduction technology."
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From TFS "This technology is fundamentally different from the "rivulet (shark skin) process," which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularit